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Using Environmental Tracers to Determine Long-Term Effects of Recharging Recycled Water in the Central Ground-Water Basin, Los Angeles County, California

By Robert Anders, and Roy A. Schroeder
U.S. Geological Survey, 5735 Kearny Villa Road, Suite O, San Diego, CA 92123

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Chlorofluorocarbons (CFCs), tritium-helium, chloride, boron, dissolved gases, and nitrogen isotopes were evaluated to determine some long-term effects of augmenting aquifer recharge using recycled water (secondary- and tertiary-treated wastewater).  The USGS collected water and gas samples from four multiple-well monitoring sites located in Los Angeles County, California, along a flowpath extending from the Montebello Forebay in the north, where water is applied to large spreading basins, to the Central Ground Water Basin in the south.  Recycled water has been used to replenish the underlying aquifers since the early 1960s.

CFCs and tritium-helium were used to date ground water with ages less than 50 years, and chloride and boron were used as tracers to locate the spatial distribution of recycled water along the flowpath.  Based on tritium-helium age determinations, ground-water samples ranged from less than 2 to more than 50 years.  Although CFCs could not be used to determine ground-water ages, they proved to be sensitive tracers for the early period of recharge with recycled water.  Using tritium-helium age determinations, in conjunction with measured CFC concentrations, researchers found that CFC concentrations are markedly elevated above atmosphere-water equilibrium in ground water older than about 20 years but still young enough to contain recycled water.  Chloride and boron concentrations are generally highest in the youngest ground water (tritium-helium age determinations).

Analysis of dissolved gases show that the redox state of the ground-water basin has changed in response to recharge using recycled water.  Dissolved-gases had higher concentrations of excess air in parts of the ground-water basin where recycled water is present.  Also, these areas have no detectable methane, whereas methane is present in the native ground water older than 50 years.  Some of the excess air probably originates as air trapped by rapid infiltration at the spreading grounds, but part has been identified as dissolved nitrogen resulting from denitrification.

In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

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